Modern combustion and gasification methods have resulted in a need for particle separators capable of efficiently and reliably separating particulate material from high-temperature combustion or product gases. In circulating fluidized bed combustion or gasification processes large amounts of dust such as solid bed material, ashes, unburnt fuel and even reactive absorbents, i.e., for sulphur capture, are continuously entrained with the exhausted high-temperature gases from the combustion chamber and recirculated back into the combustion chamber after having been separated from the hot gases. The separators used have to endure hot and corrosive conditions in the solids circulating reliability. Besides affecting the circulating fluidized bed process itself, an effective separator, by removing particulate material, minimizes fouling of heat exchange surfaces in the subsequent gas path.
In modern combined cycle power plants, where hot combustion gases are expanded in gas turbines, efficient cleaning systems for the high-temperature gases greatly extend the life and operation time of the gas turbines, since particulate matter in hot gases greatly increase erosion and fouling of turbine blades.
The present day concern with air pollution has on the other hand also created a demand for efficient dust collecting apparatuses of increased capacity, even with the capability of handling high-temperature gases.
Different kinds of particle separators for the gas cleaning processes have been proposed and constructed to achieve the needed reliable separation of solid particular matter from high-temperature and often also high-pressure gases. In recent years, gas-permeable high-temperature filters have become an interesting alternative to conventional vertical cyclone separators for separating particulate material from these high-temperature gases. Conventional vertical cyclones require substantial space both for the cyclone itself and for cyclone support constructions. Filter separators, on the other hand, can be built into substantially compact and simple systems. Filters made of porous super alloys or ceramics can endure the high-temperature conditions and do not need to be protected by cooling surfaces or refractory linings as do conventional cyclones. In pressurized processes, filter separators can very easily be integrated into the pressurized system.
In filter separators, dust accumulates gradually on the surface of the porous material and the filter has to be cleaned periodically. If large amounts of dust are allowed to accumulate on the filter surface, a large pressure difference is built up between both sides of the filter and increased pressure i.e. energy is needed for leading the gas therethrough.
Filters such as bag filters, for low temperature gases, have been cleaned by shaking, agitating, vibrating or even brushing or scraping thereby loosening dust layers accumulated on the filter medium. High-temperature rigid ceramic filters such as long, thin tube filters may be very fragile and consequently be damaged by such rough treatment.
The cleaning of rigid ceramic filters is usually arranged as reverse cleaning, by reverse flowing of clean gas through the filter medium. During reverse cleaning the main separation process in the filter separator has to be stopped in order to let the clean gas flow back through the filter. This is, of course, an undesirable interruption in continuous processes.
It has also been suggested to clean filter tubes by injecting compressed air pulses periodically into the tubes. Compressed air operates in a pulsed manner and releases dust from the filter tube surfaces. Each filter tube has to have either a compressed air injector of its own, or a common movable injector for several tubes has to be used in order to ensure cleaning of all filter tubes. U.S. Pat. No. 4,468,240 shows a filtering separator having a filter cleaning apparatus of the above mentioned type. The separator has a housing divided into a dust laden gas space and a clean gas space. Filter tubes are disposed in the gas laden space. Clean gas flows from outside into the filter tubes and out through the filter tube wall into the clean gas space. The cleaning of filter tubes is accomplished by supplying compressed air, from an injector in a reciprocatingly displaceable chamber disposed in the clean space, into the filter tubes, which are to be cleaned. The construction is very complicated and space consuming.
In U.S. Pat. No. 4,584,003 is disclosed a filter separator comprising a filter housing having vertically arranged filtration tubes and horizontally arranged filtration tube supporters dividing the filter housing into several compartments. Dust laden gas is introduced into the filtration tubes at their upper end and separated particles are discharged from their lower end. Clean gas flows through the porous tube walls into a clean gas space surrounding the tubes. Each clean space has one clean gas outlet. The filter separator is compact and simple in structure. A cleaning method for the aforementioned filter separator is disclosed in Japanese patent application No. 61-268330. A diffuser is arranged in each clean gas outlet pipe outside the filter housing. Gas from a high-pressure compressor is introduced through the diffuser throat and the clean gas outlet pipe into the clean gas space. The high-pressure gas jet also takes in circumferential clean gas from the gas outlet conduit by the ejector effect of the throat and creates a large backwashing gas stream. The main separation process does not have to be stopped during the backwashing stage.
When filtering high-temperature and high-pressure gases, the diffusers in the clean gas outlet pipes become very space consuming. The diffusers themselves are relatively large and form long projections out to one side of the filter separator. As the backwashing gas has to be of almost the same temperature as the hot gas in the filter housing to avoid thermal shocks on the filters, the diffuser and the clean gas outlet pipe have to be very well insulated, which substantially adds to the size of the elements projecting out from the filter housing. This makes the otherwise compact filter housing less attractive from a constructional point of view.